1. Field of the Invention
The invention concerns a method to generate a series of MR images to monitor a position of an intervention apparatus located in an examination region, as well as a magnetic resonance apparatus and a non-transitory, computer-readable data storage medium encoded with programming instructions to implement such a method.
2. Description of the Prior Art
Magnetic resonance MR is a known modality with which images of the inside of an examination subject can be generated. Expressed simply, the examination subject is positioned within a strong, static, homogeneous basic magnetic field (also called a B0 field) with field strengths of 0.2 Tesla to 7 Tesla and more, such that the nuclear spins of the examination subject orient along the basic magnetic field. To trigger magnetic resonances, radio-frequency excitation pulses (RF pulses) are radiated into the examination subject, the triggered magnetic resonance signals are measured, and MR images are reconstructed or spectroscopy data are determined based on these magnetic resonance signals. For spatial coding of the measurement data, rapidly switched magnetic gradient fields are superimposed on the basic magnetic field. The acquired measurement data are digitized and stored as complex numerical values in a k-space matrix. An associated MR image can be reconstructed from the k-space matrix populated with such values, for example by means of a multidimensional Fourier transformation.
For example, in medical interventions on examination subjects such as patients (such as biopsies or procedures to introduce a catheter or other artificial medical interventional devices) it is often desired to be able to monitor the course of these devices or instruments. Conventionally this is done using C-arm computed tomography systems. These use x-rays for imaging, which x-rays damage and stress both the exposed tissue and the person conducting the medical intervention. Furthermore, with C-arm computed tomography systems it is disadvantageous that the possible projection planes are limited to rotations only around a rotation axis (normally the longitudinal axis of a patient).
Therefore, there are efforts to also enable such monitoring by means of magnetic resonance. However, there are factors associated with magnetic resonance modalities that hinder a selection of the intervention devices (for example biopsy needles, catheters, etc.) that are used since these must be made of a material that is compatible with (for example) the basic magnetic field of the magnetic resonance system. Moreover, such interventional devices themselves often interfere with the excitation and/or acquisition of the measurement data, for example by inducing disruptions of the magnetic field and thus lead to artifacts (for example susceptibility artifacts) which make a precise localization of the interventional devices difficult or even impossible. This applies particularly to catheters, for example as are used in angiographies. These catheters normally are formed by plastic with guide wires made of metal, and therefore are barely visible or not visible at all in conventional MR exposures. Moreover, if monitoring takes place by means of magnetic resonance, the person conducting the interventional procedure is subjected to a high noise exposure since the typical MR examination sequences generate assessed sound pressure levels of well above 90 dB(A). Ear protectors are used to reduce the noise exposure, for example, but these are often perceived as disruptive.
In order to reduce artifacts in MR images, a significant effort is invested in the research and development of MR-compatible invention devices. In addition, it is sought to keep artifacts small by using sequences short echo times, or to process artifacts out of the data in the post-processing of the measurement data.
A number of MR-compatible markers for interventional devices are known. A selection of these is described in the United States Published Patent Application 2008/0221428 A1, for example. However, such specifically designed interventional devices are significantly more expensive and, due to the higher cost in the manufacturing, may possibly be more difficult to obtain than conventional interventional devices.
Sequences are known which enable a very short echo time. One example is the radial UTE (“Ultrashort Echo Time”) sequence, for example as described in the article by Sonia Nielles-Vallespin “3D radial projection technique with ultrashort echo times for sodium MRI: Clinical applications in human brain and skeletal muscle”, Magn. Res. Med. 2007; Vo. 57; pp. 74-81. In this type of sequence, after a wait period T_delay following a non-selective or slice-selective excitation, the gradients are ramped up and the data acquisition is begun simultaneously. The k-space trajectory that is scanned in such a manner after an excitation proceeds radially from the k-space center outwardly. Therefore, before the reconstruction of the image data from the raw data acquired in k-space can take place by means of Fourier transformation, the raw data must initially be converted into a Cartesian k-space grid, for example by regridding.
An additional approach in order to enable short echo times is to scan k-space in a point-like manner by detecting the free induction decay (FID) signal. Such a method is also designated as single point imaging since essentially only one raw data point in k-space is detected per RF excitation. One example of such a method for single point imaging is the RASP method (“Rapid Single Point (RASP) Imaging”, P. Heid, M. Deimling, SMR, 3rd Annual Meeting, Page 684, 1995). According to the RASP method, at a fixed point in time after the RF excitation, a raw data point in k-space whose phase has been coded by gradients is read out at the “echo time” TE. The gradients modified by means of the magnetic resonance system for each raw data point or measurement point, and k-space is thus scanned point-by-point as is presented in FIGS. 1a and 1b herein.